Field of the Invention
The present invention is directed to methods and apparatus for assembling large scale machine parts. Specifically, methods and apparatus are provided for lifting and torquing steam turbine throttle valve bushings in a controlled manner.
Although large scale machinery such as steam turbines are comprised of smaller sub-assemblies and parts which are assembled using conventional fasteners such as threaded connections, the size and bulk of these parts makes assembly an extremely difficult task. For example, a steam turbine throttle valve bushing which is threaded into a throttle valve bonnet weighs about 120 pounds (54 kg). The bushing has a 5.50-12 UN thread, with three close fitting lands below the thread. To assemble the bushing into the bonnet, it must be carefully lowered, the threads aligned, and tightened to an appropriate torque.
Unfortunately, this type of delicate assembly operation is not easily performed by the cranes and other apparatus which are typically used to manipulate such parts. As the bushing is being lowered, it will contact the threaded portion of the bonnet. At this point it is difficult to control most material handling equipment with sufficient precision to prevent the full weight of the bushing from bearing on the bonnet threads. This is not a desirable situation, for several reasons. First, the parts may contact one another in a manner which leads to damage of the threads. The parts may also be brought together in a manner which leads to cross-threading, particularly if the threaded portions are not clearly visible and the size of the part makes if difficult to "feel" if cross-threading has been initiated. Also, even if the parts are initially assembled in a proper alignment, the friction created by the weight of the parts may lead to binding or other difficulties which make accurate torque measurements difficult.
Therefore, it would be desirable to be able to provide a resilient connection between materials handling equipment and the part being handled. It would be further desirable to provide methods and apparatus whereby parts may be guided to a location in a controlled manner, avoiding the problems created by impact and imprecise handling, while allowing subsequent final assembly to a specified torque in an efficient manner.